With the recent development of the atomic energy industry, practical studies on electrodes for uranyl ion analysis that are essential to the atomic energy industry have been proceeded. With respect to uranyl ion selective electrodes of liquid ion-exchange membrane type, in which a uranyl ion inner solution at a fixed concentration is filled in the space between a reference electrode placed in the inner portion thereof and an outer tube, various reports have been made.
For example, it was reported in W. C. Dietrich, Technical Progress Report, No. Y1174D Y-12, Development Division (August-October, 1971) that a poly(vinyl chloride) (PVC) membrane containing a complex of di-(2-ethylhexyl) phosphate (D2EHP) and a uranyl ion is responsive to uranyl ions.
D. L. Manning et al. studied properties of PVC-based responsive membranes containing a uranyl ion complex formed with an acidic phosphoric ester using an acidic or neutral phosphonic ester or phosphoric ester as a diluent and proposed various kinds of PVC-based responsive membranes based on their study. However, a slope of potential response with respect to a common logarithmic concentration that is correlated to concentration responsiveness (hereinafter referred to as slope) of their responsive membranes was 26 mV/decade at the highest, that is smaller than the theoretical slope for divalent ions according to the Nernst equation (hereinafter referred to Nernstian slope (see Anal. Chem., Vol. 46, No. 8, pp. 1116-1119 (1974)).
I. Goldberg et al. studied properties of PVC-based responsive membranes made of a uranyl ion complex formed with an acidic or neutral phosphoric ester, a phosphorous ester or an acidic phosphonic ester using any one of D2EHP, tributyl phosphate (TBP) and a variety of phosphonic esters as a diluent and obtained a slope close to the Nernstian slope from phosphorous ester type responsive membranes (see Anal. Chem., Vo. 52, No. 13, pp 2105-2108 (1980)).
According to the above-cited reports, neither Manning et al. nor Goldberg et al. tried to mix PVC with a solvent mediator.
In general, electrodes having a greater slope exhibit higher performances as sensors. Any of the conventional responsive membranes as reported did not succeed to attain a slope greater than the Nernstian slope for divalent ions and are, therefore, still unsatisfactory for practical use as uranyl ion selective electrodes.